Abstract: Oxides with the bixbyite structure have two crystallographically unique cationsites, namely (in Wyckoff notation) 24d and 8b. Since the symmetries of these two sites are different (C2 and S6, respectively), properties related to solute cations will vary depending on the site preference. Therefore, we have employed atomic scale simulation techniques to systematically investigate the solution site preference of a range of trivalent cations ranging from Sc3+ to La3+ in A2O3 bixbyite oxides (where A ranges from Sc to La). Results reveal that when the solute cation is smaller than the host lattice cation, the 24d site is energetically favorable, but when the solute cation is larger than the host lattice cation, the 8b site is preferred. We also discuss the tendency for solute cations to cluster, as well as corroboration of this work by first principles methods.

Abstract: Building upon work in which we examined defect production and stability in spinels, we now turn to defect kinetics. Using temperature accelerated dynamics (TAD), we characterize the kinetics of defects in three spinel oxides: magnesium aluminate MgAl2O4, magnesium gallate MgGa2O4, and magnesium indate MgIn2O4. These materials have varying tendencies to disorder on the cation sublattices. In order to understand chemical composition effects, we first examine defect kinetics in perfectly ordered, or normal, spinels, focusing on point defects on each sublattice. Wethen examine the role that cation disorder has on defect mobility. Using TAD, we find that disorder creates local environments which strongly trap point defects, effectively reducing their mobility. We explore the consequences of this trapping via kinetic Monte Carlo (KMC) simulations on the oxygen vacancy (VO) in MgGa2O4, finding that VO mobility is directly related to the degree of inversion in the system.

Abstract: We show that parallel replica dynamics can be extended to driven systems (e.g., systems with time-dependent boundary conditions). Each processor simulates a replica at a driving rate that is M times faster than the desired rate, where M is the number of processors. As in regular parallel replica dynamics, when a transition to a new state is detected on any processor, the times are summed and every processor is restarted in the new state. The state-to-state dynamics are shown to be correct if the processors run at the same speed and the system is driven slowly enough (on each processor) so that the escape rates do not depend on the time history of the drive. We demonstrate the algorithm by stretching a carbon nanotube with a preexisting vacancy, noting a significant dependence of the nature of nanotube yield on the strain rate. In particular, we are able to achieve strain rates slow enough such that the time scale for vacancy diffusion is faster than that for mechanical yield at a temperature of 2000 K. We thus observe vacancy-induced morphological changes in the nanotube structure, providing some insight into previously unexplained experimental features.